Occasionally when you've pasted into Vim from some other application, you get a pesky visible non-ascii character. You can do a ga command to see what it is, but you might want to search for it or substitute it.

:help i_CTRL-V_digit (which applies also on the command-line, notwithstanding its i_ initial) about additional ways to enter a character by its hex value (below 0x100: by its hex, decimal, or octal value)